Gemology and Chrysology

Summary

Gemology is the study of minerals that are attractive enough to be worn as jewelry after being cut and polished. Gemstones are often transparent, internally reflect light, are colored, and have a brilliant luster. Sometimes, the term gemstone is also used to refer to attractive aggregates of minerals (such as turquoise), organic materials (such as amber), rocks (such as marble), or synthetically made materials. Only about seventy of the many thousands of known minerals have varieties that are of gem quality. Chrysology is the study of precious metals such as gold, silver, and platinum. Precious metals are opaque and often have colors (such as the yellow of gold) that make them attractive in jewelry.

Definition and Basic Principles

Gemstones must be attractive to viewers. Their attractiveness depends on the color, transparency, and luster of the gem and the way it is cut. The "big four" are the best-known and most valuable gemstonesdiamonds, sapphires, rubies, and emeralds. Diamonds are often yellowish and transparent, with a brilliant adamantine luster. Some of the most valuable diamonds contain more intense green, red, blue, or black colors. Their rarity often makes them more valuable.

Gemstones ideally should be harder than the common mineral quartz and should not fracture easily so that the gemstone is not easily scratched or broken when worn as jewelry. About ten minerals have these characteristics. Diamonds are the hardest known minerals, but they can be fractured along cleavage planes if excessive force is applied. Gemstones, softer than quartz, retain their luster and beauty as long as they are not scratched. Valuable gemstones should not contain many small impurities, such as minerals or fractures, that might degrade the gem's beauty.

Gemstones are often cut in certain ways to enhance their beauty. The choice of cutting method depends in part on the characteristics of the stone. Diamond, for instance, has four major planes of cleavage (octahedral cleavage) along which the diamond may be broken. Diamonds are cut along these octahedral cleavage planes so that light is internally reflected within the diamond, producing many flashes of light as the gem is viewed.

In contrast to gemstones, precious metals such as gold and silver are opaque and have a metallic luster. Their beauty comes from their color in reflected light.

Background and History

Gemstones and precious metals are mentioned in ancient writings from cultures around the world, and they have been found in many archaeological digs, suggesting that they have been used for thousands of years. Ancient Babylonians had gemstones of jasper (red, fine-grained quartz) and lapis lazuli (blue, a mixture of the mineral lazulite and some other minor minerals).

Many gemstones were found in Egyptian tombs dating from 5000 to 3000 BCE. These gems include lapis lazuli, agates (silica-rich mineral), jasper, and emeralds (dark green, transparent beryl, a beryllium aluminum silicate). Some of these gems may be traced to specific geologic sources, suggesting that trading occurred over large areas. An ancient Egyptian papyrus contains a map showing the location of several gold mines and the miners' quarters. Gold was likened to the skin of the gods, and it was used in funerary art and worn only by royalty.

Many gemstones were worn not only because of their beauty but also because many believed that the gems could protect them from evil forces and diseases and make them wise. Astrologers claimed that certain gems gave the wearer special powers in certain months. The Bible mentions a variety of gemstones, including emeralds, sapphires (blue, transparent corundum, an aluminum oxide mineral), diamonds, agates, and amethysts (purple, transparent quartz), as well as gold and silver, which were sources of wealth and used for decoration.

How It Works

Traditional Identification Methods. Most gemstones and precious metals are minerals or are found in minerals, so the classic methods of identifying minerals via physical properties may be used. These physical properties are crystal forms (such as cubes or octahedrons), color, hardness, luster (metallic or nonmetallic), cleavage, density, and microscopic characteristics. Experienced gemologists can identify a gemstone if they can determine its luster, color, and other features by visual inspection, using no more than a magnifying glass. Some properties, such as hardness, may be of limited use to identify a valuable gemstone, however, because checking for hardness could damage the specimen.

Some specialized techniques may be used to identify a gemstone. Determining the gemstone's density can be very useful if the gemstone is not mixed with other minerals and is not mounted in jewelry. A gemologist might place the gem in a series of liquids of known densities. If the gemstone is denser than the liquid, it will sink. If it is less dense, it will float. Most minerals have very different densities, so this process can confirm the identity of the material.

Identification Using Instruments. A binocular microscope with a ten- to sixty-power magnification may be used to observe a gemstone using light reflected from or transmitted through the specimen. Imperfections within a gemstone, for instance, can be seen using transmitted light.

The refractive index of a gemstone may be obtained using a refractometer. Most minerals have a unique refractive index, so this determination can also help confirm the identity of the gemstone. Refractive indices commonly range from a low of 1.4 to a high of 3.0. A gemstone typically should have one flat face that is placed on the stage of the refractometer in immersion oil with a refractive index of 1.81. The mineral and light source are moved relative to each other until the light is internally reflected, and the refractive index is read directly on a scale. Some minerals have only one refractive index, so the readings are straightforward. Others have more than one refractive index in different directions, so determining these values is more challenging.

Another approach to determining the refractive index is to place a gemstone in a series of liquids of known and different refractive indices and observe them under a microscope. If the refractive index of the gem is the same as the refractive index of the liquid, then the mineral will nearly disappear. If the refractive index of the gem is substantially different from the refractive index of the oil, then the mineral edges will stand out in bold relief.

If these methods fail to identify a gemstone, then other, more destructive techniques, such as X-ray diffraction, electron microprobe analysis, and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), might be used for identification. Often these techniques use material that has been scraped off parts of the gemstone that will not be exposed to a viewer. X-ray diffraction techniques can be especially useful in confirming the identity of a mineral because each mineral presents a unique pattern.

Applications and Products

The most important gemstones are varieties of diamond, corundum (aluminum oxide; sapphires and rubies), beryl (beryllium aluminum silicate; especially emeralds), tourmaline (sodium calcium, lithium silicate borate hydroxide mineral), spinel (magnesium aluminum oxide), topaz (aluminum silicate hydroxide), and quartz (silica, such as amethyst).

Diamond, because of its hardness, high refractive index, numerous colors, rarity, and ease of cutting, is typically considered the most important gemstone. Diamonds are the most common gemstones in jewelry stores. Most diamonds are not of gem quality, but they can be used for industrial purposes, such as in cutting materials. Most diamonds have a yellow tinge, but some rare diamonds with few imperfections may have intense green, blue, gold, red, or black colors that make them very valuable. The largest diamond ever found was the Cullinan, at 3,106 carats. It was cut into smaller diamonds, the most famous of which is the colorless 530-carat Star of Africa diamond.

Rubies are the red gemstones of corundum. Sapphires can be blue, yellow, green, or violet. Only diamonds are harder than rubies or sapphires. Rubies and sapphires are difficult to cut because they have no cleavage and are brittle. Some of the most famous rubies and sapphires are much more expensive per carat than diamonds. One of the most famous sapphires is the Star of India (563 carats), and one of the most famous rubies is the Edwards ruby (167 carats).

Beryl has many colors, but dark green transparent emeralds are the most expensive. Blue beryls of gem quality are called aquamarines. Emeralds are known for growing particularly large. One of the largest named emeralds is the Sacred Emerald Buddha, a 2,620-carat statue carved from a 3,600-carat emerald crystal in 2006.

Tourmaline, spinel, and topaz occur in many colors. Pink and green varieties of tourmaline are the most popular, and the red varieties of spinels are the most valued. Good specimens of pink topaz are the most expensive.

Quartz is usually transparent and colorless, but it may also be violet, black, blue, yellow, pale green, or pink. Amethyst is the name for violet quartz, which is generally more valuable than the other varieties of quartz.

Lesser Gemstones. Many other gemstones of lesser importance are commonly used. For example, spodumene (lithium aluminum silicate) can occur as transparent yellow, green, or colorless specimens. Turquoise (calcium aluminum silicate) is a blue-green opaque mineral that is usually mixed with other dark impurities or some copper minerals. It has a hardness less than that of quartz and a good cleavage, so care must be taken not to damage it.

Artificial and Lab-Grown Gemstones. Artificial or synthetic gemstones have been produced since the early part of the twentieth century. They are often difficult to distinguish from natural gemstones, thereby somewhat diminishing the demand for some natural gemstones. Artificial gemstones can be made in several ways. Some are produced by precipitation of the gems from a hot steam solution. Others are created by melting, at the appropriate temperature and pressure, a mixture of components in a ratio that approximates that of the natural gem they will resemble.

Since the 1950s, synthetic diamonds have been created by subjecting carbon to very high temperatures and pressures. Most of these diamonds are not of gem quality but can be used as abrasives or for other industrial purposes. Artificial diamonds can be distinguished from natural diamonds by the presence of certain impurities, such as nitrogen or boron, used in their manufacture.

By the 2020s, creating real diamonds in lab settings by replicating the natural "growing" process of the gemstones had become more regular and commercialized, particularly in the jewelry industry. Some gemstone sellers specialize in lab-grown diamonds, using carbon atoms that grow diamonds through technology and controlled environments as a more ethical and affordable option for acquiring gemstones.

Precious Metals. Gold and silver have been used for thousands of years both as a medium of exchange and for ornamentation. They were used to make plates, cups, drinking vessels, ornamental objects, religious icons, and jewelry. Gold and silver have also been used in applications such as dentistry, electronics, and engineering. The refining of platinum was not perfected until the nineteenth century, so fewer applications have been developed. Platinum began to be used more commonly in jewelry in the 1960s, and it is also used as a catalyst in some chemical reactions.

Careers and Course Work

A career in gemology or chrysology begins with a certification program or a bachelor's degree in geology or mining engineering, with coursework in mineralogy and gemology. A doctorate in one of these fields is typically required for researching gemstones or precious metals. Additional coursework in chemistry and physics may be helpful for individuals interested in being a lab gemologist. Those interested in manufacturing, appraising, designing, and cutting gemstones should seek classes in gem faceting. Research involving applications that use gemstones or precious metals requires coursework and degrees in those areas, such as medicine or electronics.

Those desiring to work as a gemstone identifier and grader in a jewelry store or as a jewelry maker or designer can obtain specialized training in these areas. The Gemological Institute of America (GIA) offers a graduate gemologist degree and a jewelry manufacturing arts degree, both six-month programs. The International Gem Society offers a professional gemologist certificate program and a diamond specialist program. Much of the knowledge needed to succeed in gemology and chrysology is acquired on the job.

Social Context and Future Prospects

The demand for gemstones is likely to continue, rising and falling with upturns and downturns in the economy, keeping gemology relevant. The United States was the biggest market for gemstones entering the 2020s, driving over a third of global consumption. Asian markets also experienced strong growth in the early twenty-first century.

Gemstones, like many minerals, are found in only a limited number of locations worldwide, so the supply of a given gemstone can vary considerably. A major mine may become depleted, or a new source of the gemstone may be discovered. Diamond prices were historically the most stable because the market was long dominated by the corporation De Beers. However, other diamond producers increased their presence in the market in the twenty-first century, particularly Russian companies such as ALROSA, introducing more competition and price fluctuation. The best rubies and sapphires are so rare that the price per carat for individual specimens can exceed that for diamonds. Myanmar is considered the source for the best rubies and sapphires. Colombia has traditionally produced the finest emeralds.

Much like gemstones, precious metals have remained consistently popular with consumers for jewelry. As commodities, precious metals are also influenced by factors such as limited supply and industrial use. Prices can fluctuate considerably over relatively short periods, especially as many investors view precious metals as safe in times of turmoil. Platinum is also valuable in the automotive industry as a component of catalytic converters. When demand for new vehicles slid in 2020 and 2021 during the COVID-19 pandemic, platinum was in ample supply but low demand and its value fell accordingly. The American diamond industry also saw a significant rise in the sale of lab-grown diamonds through the late 2010s and 2020s.

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